1. Field of the Invention
This invention is directed to roof support fixtures for underground passages in which the support fixture develops a controlled yield in response to movement of the surrounding material in which the passage has been created.
2. Description of the Prior Art
It is known that the creation of underground passages releases forces in the geologic mass which has been initially substantially in balance before the passage was formed. Means for resisting the forces to reestablish a balance so as to retard the movement of the geologic mass surrounding the passage have been proposed in several different forms.
An early form of roof bolt is disclosed in Ralston U.S. Pat. No. 2,850,937 of Sept. 9, 1958. In this disclosure, the roof bolt provides indicator means which can be seen and which conveys information regarding whether or not the roof bolt is supporting its desired load in a mine ceiling.
The disclosure by Reusser et al in U.S. Pat. No. 3,478,523 of Nov. 18, 1969 is directed to a load bearing plate for use in mine roof supports intended to cover relatively small or localized surfaces of the mine roof to stabilize the rock formation and prevent collapse thereof.
The Karara U.S. Pat. No. 3,693,359 of Sept. 26, 1972 is directed to rock stabilizing apparatus in which a tension member inserted in a hole and cemented therein carries a tapered wedge slidable on the tension member and held by a threaded element.
The prior art includes devices for visually indicating the movement of the roof in mine passages, such art being exemplified by Koski U.S. Pat. No. 2,725,843 of Dec. 6, 1955; Curry U.S. Pat. No. 2,943,528 of July 5, 1960; Hohos et al U.S. Pat. No. 2,947,279 of Aug. 2, 1960; and Harrison U.S. Pat. No. 3,161,174 of Dec. 15, 1964.